1. Field of the Invention
This invention relates to a method applied to reservoir geophysics for delineation of the reservoir architecture of oil or gas fields and for mapping the continuity of producing horizons as well as correlating lithology and stratigraphy between wells. The architecture of a reservoir generally includes structures of flow units, ceiling surfaces and boundary surfaces that result from deposition and digenesis. In particular, this invention provides a method for detecting the presence of continuous and discontinuous low-velocity inclusions in subterranean geological formations by analyzing seismic signatures recorded between two or more wells or boreholes, and determining the distance of a discontinuity with respect to the position of a receiver borehole.
2. Description of Prior Art
It has long been desirable to analyze and evaluate structure and stratigraphy of petroleum producing formations. One purpose, for example, has been in conjunction with secondary and tertiary recovery operations. In recent years, there has been increased interest in seismic methods for reservoir definition and description. One method has been the crosswell seismic method in which seismic energy emitted from sources in one well or borehole is sensed and recorded as seismic data by arrays of detectors in one or more other wells. The recorded seismic data is then processed to form tomographic images of interwell geologic features based on the crosswell seismic data. Techniques using crosswell seismic data are based on tomographic reconstruction for interwell seismic velocity imaging. Although tomographic imaging between boreholes can produce very good images, certain conditions must be present. Unfortunately, in most petroleum reservoirs, these conditions are quite difficult to achieve. U.S. Pat. No. 5,144,590 teaches a method for determining the continuity of subsurface formations between well boreholes in which seismic energy is sent from a seismic source in one of the wells at various selected fixed depths and detected by a number of sensing geophones deployed at selected fixed depths in one or more adjacent wells. A frequency domain decomposition process is then performed on the data in order to determine if any of the formations located between the wells function as waveguides for seismic energy within the frequencies of interest. Those formations exhibiting waveguide properties are indicated as continuous between the wells.
U.S. Pat. No. 5,005,159 teaches a method of determining the continuity of a lithographic layer located between two vertical boreholes in which a seismic source is lowered into the first borehole while simultaneously a receiver pair, preferably a pair of "vertical" geophones spaced apart by about two feet, are lowered into a second borehole to develop a "difference signal". The recording of high amplitude signals within a layer is an indication of a continuous, low velocity layer. The middle of a layer can also be discovered using a single vertical receiver and finding the place of phase reversal. See also U.S. Pat. No. 4,783,771, U.S. Pat. No. 4,751,688, U.S. Pat. No. 5,197,038, U.S. Pat. No. 5,253,217, U.S. Pat. No. 5,260.911, and U.S. Pat. No. 5,200,928, all of which relate to methods of seismic surveying for defining subterranean formations. See also U.S. Pat. No. 5,481,501 which teaches a method for simulating crosswell seismic data between first and second spaced apart wells and U.S. Pat. No. 5,648,937 which teaches a method and apparatus for adjusting the results of a seismic survey according to well log data obtained from wells within a survey region.
State of the art crosswell technology does not currently address the presence of different trapped modes and body waves associated with the petrophysics of an inclusion and host rock. In addition, the current methodology does not utilize the response characteristics of individual seismic events observed in full waveform signatures as recorded at receiver boreholes to determine if a low-velocity inclusion is continuous or discontinuous. Accordingly, a method is needed for planning continuity logging surveys using lithological information, for example, compressional and shear wave velocity logs, and density logs, as input parameters to predict full waveform seismic signatures at different well separations and inclusion thicknesses. A method is also needed which enables prediction of continuous and discontinuous inclusions through analysis of the different seismic responses or (vents recorded between wells.